Method for recognizing a change in lane of a vehicle

ABSTRACT

A method of detecting a lane change of a subject vehicle ( 20 ), having a locating device ( 10 ) which uses angular resolution for locating vehicles (VEH 1 , VEH 2 , VEH 3 ) traveling in front, and a device ( 44 ) for determining the yaw rate (ω 0 ) of the subject vehicle. The angular velocity (ω i ) of at least one vehicle traveling in front relative to the subject vehicle ( 20 ) is measured using the locating device ( 10 ), and a lane change signal (LC) indicating the lane change is formed by comparing the measured angular velocity (ω i ) to the yaw rate (ω 0 ) of the subject vehicle.

BACKGROUND INFORMATION

The present invention relates to a method of detecting a lane change ofa vehicle having a locating device which uses angular resolution forlocating vehicles traveling in front, and a device for determining theyaw rate of the subject vehicle.

Distance- and speed-regulating devices for motor vehicles, also referredto as ACC (adaptive cruise control) systems, are known. In thesesystems, objects, for example vehicles traveling in front in the samelane as the subject vehicle, are detected using a locating device, forexample a radar system which provides angular resolution, which enablesthe distance and relative speed of the vehicle traveling in front to bemeasured. The capability for angular resolution in such a radar systemhas been used in the past to check the detected objects forplausibility, so that, for example, vehicles in the same lane as thesubject vehicle may be distinguished from road signs or markings on theedge of the roadway, or from vehicles in other lanes.

When a vehicle traveling in front in the same lane as the subjectvehicle is within the locating range of the radar, the traveling speedis regulated by intervention in the drive or braking system of thevehicle in such a way that a speed-dependent distance from the vehicletraveling in front is maintained. On the other hand, if there is novehicle within locating range in the same lane as the subject vehicle,the device regulates the speed at an intended speed selected by thedriver.

German Patent Application 196 37 245 A1 describes an ACC system in whichthe evaluation of the radar signal for plausibility is modified when thedriver indicates his/her intention to change lanes by actuating the leftor right turn indicator. In this situation, the travel corridor takeninto account in regulating the distance is temporarily extended to thefuture new lane, and the vehicles in the former lane as well as thevehicles in the future lane are taken into account in regulating thedistance. The travel corridor is defined as a strip of fixed, oroptionally variable, width on both sides of the prospective travel pathof the subject vehicle. For a straight roadway course, the travel pathof the subject vehicle is indicated by a straight line running in thedirection of travel through the center of the vehicle. For a curvedroadway course, it may be assumed as an approximation that theprospective travel path is a curve of constant curvature. Assuming asteady-state curve situation, the particular curvature may be calculatedby defining the yaw rate of the subject vehicle via the traveling speed.In principle, the yaw rate may be determined from the steering angle andthe traveling speed, but preferably is directly measured using a yawrate sensor, in particular since such a yaw rate sensor is alreadypresent in vehicles having an electronic stability program (ESP).

In non-steady state situations, however, in particular during a lanechange, an accurate determination of the travel corridor has proven tobe difficult. Merely evaluating the signal from the turn indicator is ofno further use here, since actuation of the turn indicator onlyindicates the intention to change lanes but does not allow the detectionof exactly when the lane change starts and ends. Even making additionalallowance for the steering commands of the driver does not enable thelane change to be unambiguously detected, since a curved roadway coursemay also give rise to the steering commands. In the past, theseuncertainties in the detection of a lane change have often causedmalfunctions in the regulating system due to the fact that during thelane change the radar beam temporarily sweeps over the edge of theroadway and identifies stationary targets such as road signs or the likeas presumably relevant objects, or that for roadways having three ormore lanes, vehicles in the next-to-adjacent lane are erroneouslyassociated with the travel corridor of the subject vehicle. Toaccurately associate objects detected using the locating device with therelevant travel corridor of the vehicle, it would therefore be desirableif a lane change could be reliably identified.

OBJECT, ACHIEVEMENT, AND ADVANTAGES OF THE INVENTION

The object of the present invention is to provide a method which allowsa lane change to be more accurately detected.

This object is achieved according to the present invention by the factthat the angular velocity of at least one vehicle traveling in frontrelative to the subject vehicle is measured using the locating device,and a lane change signal indicating the lane change is produced bycomparing the measured angular velocity to the yaw rate of the subjectvehicle.

The present invention is based on the concept that, during a lanechange, in contrast to traveling along a curve, there is a distinctnegative correlation between the relative angular velocity of vehiclestraveling in front and the yaw rate of the subject vehicle. This is dueto the fact that at the start of a lane change the subject vehicleundergoes a yawing motion, and thus a rotation about the vertical axis,with a relatively high yaw rate, i.e., a relatively high angularvelocity, whereas the objects detected by the locating device do nottake part in this rotation and therefore have an angular velocityrelative to the subject vehicle which is equal in terms of actual amountbut opposite in direction. When traveling through a curve of constantcurvature, however, the subject vehicle and the vehicles traveling infront—at the same traveling speed—undergo the same rotation, so that therelative angular velocity of the vehicles traveling in front remainsapproximately zero. Only during travel into or out of a curve is itpossible for a definite difference between the relative angular velocityof the vehicle traveling in front and the yaw rate of the subjectvehicle to appear, although these differences generally are considerablysmaller than those during a lane change. Comparing the relative angularvelocities to the yaw rate of the subject vehicle thus provides a veryreliable criterion for detecting a lane change.

Advantageous embodiments of the present invention result from thesubclaims.

Since at higher traffic densities multiple vehicles traveling in frontare generally simultaneously detected by the locating device, it ispreferable to form a composite angular velocity from the measuredrelative angular velocities of several or all of the detected vehicles,for example by forming an average, or a weighted average based on thedistance or angle. By assigning greater weight to vehicles which haveonly a slight angular deviation from the path of the subject vehicle, itis possible to reduce interference effects caused by the relative speedsof the vehicles traveling in front. Similarly, by assigning greaterweight to vehicles only a small distance from the subject vehicle,interference effects which appear when entering into a curve arereduced. However, the noise from the angular signal of vehicles in closeproximity is generally increased because of the motion of these samevehicles. To suppress such noise, in addition to information on thevehicles it is usually possible to also collect time-specificinformation. Since the radar measurements are typically repeatedperiodically in a fixed regulating cycle, information is provided overmultiple regulating cycles, so that here as well, a lower weight may beassigned to the older cycles.

In addition, a plausibility effect may occur during the determination ofthe composite angular velocity. For example, for three or more vehicleswhich are being localized it may be practical to eliminate outlierswhose angular velocity clearly deviates from the other vehicles. Thus,it is possible in particular to reduce interference effects caused by alane change of one of the vehicles traveling in front. If only twovehicles traveling in front are within the locating range, generally alane change by one of the vehicles traveling in front is assumed only ifone of these vehicles starts to pass or completes the passing maneuver.These situations may be identified using data on the measured distanceand relative angular velocity.

Based on similar considerations, it may be advantageous for vehicleswhich have just appeared in the locating range because they have passedthe subject vehicle not to be included in the calculation of thecomposite angular velocity unless a certain time delay has occurred.

In the determination of the relative angular velocities of theindividual vehicles, it may be useful to apply a correction due to therelative speeds of these vehicles. For example, a vehicle which has justbeen passed by the subject vehicle has a relative angular velocity thatis different from zero, without this indicating that the subject vehiclehas changed lanes. This relative angular velocity is proportional to theproduct of the relative speed and the angle at which the vehicle islocalized, divided by the distance of this vehicle, and may beeliminated by subtracting an appropriate correction factor.

If the composite angular velocity ω_(c) of the vehicle traveling infront and the yaw rate ω₀ of the subject vehicle were determined, asignal LC is obtained which indicates with high reliability a lanechange of the subject vehicle by forming the negative of thecross-correlation value of these variables: LC=−ω₀*ω_(c)/(ω_(c)+ω₀). Assoon as this signal exceeds a specified threshold value, it can beassumed that a lane change of the subject vehicle is occurring.

Optionally, the signal from the turn indicator may also be taken intoaccount in such a way that when the turn signal is actuated, thethreshold value to which signal LC is compared is decreased. It is alsopossible to distinguish whether the left or the right turn indicator wasactuated, so that the threshold value is decreased only when the lanechange occurs in the correct direction. The direction of the lane changeis specified by the algebraic sign of ω₀.

Yaw rate signal ω₀ may also be checked for a pattern which is typical ofa lane change in order to increase the reliability of the information.During a lane change this: signal exhibits a characteristic S-shapedcurve. According to a further embodiment of the present invention, theexpected completion of the lane change as well may be predicted fromthis pattern. Alternatively, it may be assumed that the lane change iscompleted when a certain time period, which optionally is speed-related,has elapsed after the lane change is detected.

The lane change signal thus obtained may be used within the scope of anACC system and in many other ways as well. In particular, the travelcorridor of the subject vehicle may be appropriately adjusted duringdetection of the start of a lane change. It is also possible to takeinto account that at the midpoint of the lane change the direction oftravel of the subject vehicle deviates from the direction of theroadway. The value of this angular deviation may be quantitativelydetermined by integrating the yaw rate signal, the composite angularvelocity signal, or a combination of both over time, and may then beused to correct the predicted travel path and thus the travel corridor.In this manner it is possible to prevent the erroneous evaluation ofstationary targets during the lane change. In one even simplerembodiment, this effect may also be achieved by reducing the penetrationdepth of the locating device so that objects farther away continue to bedisregarded in the distance regulation.

In addition, the lane change signal may be used to temporarily extendthe travel corridor to the adjoining lane which forms the future lane,and to narrow the travel corridor back to the new lane after the lanechange is completed. Likewise, it is possible to use the lane changesignal to trigger certain additional functions which are implemented inthe ACC system, for example a passing aid which assists in merging intothe flow of traffic in the future lane by automatically accelerating ordecelerating the vehicle. The lane change signal may also be evaluatedfor special functions besides the actual ACC regulation, for example forlighting control which automatically adjusts the beam direction from theheadlights of the vehicle.

Regulating systems are also known which detect the course of the roadwayby evaluating a camera image or by using other sensors, and which assistthe vehicle in staying within the lane (lane keeping support) byintervening in the steering of the vehicle. When the vehicle is equippedwith such a system, it is possible not only to detect the lane changedirectly by evaluation of the sensor signals which sense the edge of theroadway, but in this case also to use the method according to thepresent invention for plausibility testing.

BRIEF DESCRIPTION OF THE DRAWING

An exemplary embodiment of the present invention is described in moredetail below with reference to the drawing.

FIG. 1 shows a block diagram of a distance- and speed-regulating systemfor motor vehicles which is designed to carry out the method accordingto the present invention;

FIG. 2 shows a diagram of a three-lane roadway having travel corridorsin which vehicles traveling in front which are relevant to the distanceregulation are situated;

FIG. 3 shows a diagram corresponding to FIG. 2 which illustrates a lanechange of the subject vehicle;

FIG. 4 shows the time curve of various variables which characterize thelane change illustrated in FIG. 3;

FIG. 5 shows a diagram of a driving situation in which the regulatedvehicle and a vehicle traveling in front are driving out of a curve; and

FIG. 6 shows the time curve of the same variables as shown in FIG. 4 forthe driving situation illustrated in FIG. 5.

DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

Because the design and operating principle of a distance- andspeed-regulating system, referred to below as an ACC system, are knownas such, FIG. 1 shows only those components of such a system that areimportant for understanding the present invention.

A radar sensor 10 is provided as a locating device for vehiclestraveling in front, and is mounted on the front of the regulated vehicleand periodically locates target objects situated in front of thevehicle, for example vehicles traveling in front, and stationary targetson the edge of the roadway. By evaluating the radar echo, signals areproduced, either in the radar sensor itself or in a processing unitconnected downstream, which indicate the distance d_(i), the relativespeeds v_(i) (in the radial direction), and the azimuth angles ψ_(i) ofthe located objects. The azimuth angles here are defined with respect tothe instantaneous straight-ahead direction of the vehicle. Positiveazimuth angles correspond to an angular deviation in the mathematicallypositive sense, and thus to the left.

An electronic regulating device 12 evaluates the data sent by radarsensor 10 and intervenes in the drive system and, if appropriate, alsoin the braking system of the vehicle in order to regulate the speed ofthe vehicle to maintain a suitable, speed-dependent distance from thevehicle traveling immediately in front in the same lane as the subjectvehicle. If no vehicle traveling in front is localized, the deviceregulates the speed at an intended speed selected by the driver.Stationary targets on the edge of the roadway are differentiated fromvehicles traveling in front based on the angular signals and therelative speed. Since the ACC system is provided primarily for use onmultilane freeways and highways, the lane in which the vehicles aresituated must also be distinguished for vehicles traveling in front.Normally, only the vehicles in the same lane as the subject vehicle aretaken into account for the distance regulation.

FIG. 2 shows as an example a one-directional roadway having three lanes14, 16, and 18. A vehicle 20 equipped with the ACC system according toFIG. 1, referred to below as the “subject vehicle,” travels in rightlane 14, and vehicles 22, 24, 26, and 28 traveling in front are situatedin lanes 14 and 16. For the distance regulation, only the data fromvehicles 22 and 24 situated within a limited distance range in a travelcorridor 30, which ideally coincides with lane 14, are taken intoaccount. Travel corridor 30 is defined as a strip of predetermined widthon both sides of path 32 which is expected to be followed by subjectvehicle 20, and is indicated by a dotted-dashed straight line in FIG. 2.In the example shown, a straight roadway course and a correspondingstraight-line path 32 are present. Methods for predicting the travelpath on a curved roadway are known as such, and will not be describedhere in further detail. To decide whether a vehicle is situated withintravel corridor 30, a path offset y is determined for each localizedobject, and a check is performed as to whether, in terms of actualamount, this path offset is less than a threshold value corresponding toone-half of the typical width of a lane. Path offset y, which in FIG. 2is shown for vehicle 26, may be calculated from measured distance d andazimuth angle ω of the affected vehicle, and corresponds approximatelyto the product d*ψ.

If the driver of subject vehicle 20 decides to change to middle lane 16,vehicles 26 and 28 situated in travel corridor 34 corresponding to theadjoining lane are also to be taken into account for the distanceregulation. After the lane change is completed, if subject vehicle 20 istraveling approximately in the middle of lane 16, only travel corridor34 is of significance, which however is then defined by the same pathoffset y as was travel corridor 30 originally. During the lane change,subject vehicle 20 temporarily changes its direction relative to lanes14, 16, so that prospective path 32 which is defined by thestraight-ahead direction of the vehicle no longer corresponds to theactual course of the roadway.

To enable a consistent distance regulation, even during a lane change,and to avoid malfunctions that may irritate the driver or causediscomfort, a method is described here which allows the beginning andalso the completion of a lane change to be automatically detected.

As shown in FIG. 1, signals ψ_(i) sent by distance sensor 10 whichindicate the azimuth angle of the localized objects are led to adifferentiation element 36 which calculates the associated relativeangular velocities ω_(i). In practice, this may be carried out so thatthe azimuth angles measured in successive regulating cycles aresubtracted from one another, and the difference is divided by theduration of the regulating cycle (in the range of 1 ms). To suppressnoise effects, the raw data thus obtained may subsequently undergolow-pass filtering using a suitable time constant of 0.5 s, for example.

Filtered relative angular velocities ω_(i) are then corrected forrelative speed-dependent effects in a correction module 38. The natureand purpose of this correction are explained below.

Corrected relative angular velocities ω′_(i) are linked in a logiccircuit 40 to form a composite angular velocity ω_(c) which represents ameasure of the change in the angle of the overall composite of allvehicles 22, 24, 26, and 28 traveling in front, relative to subjectvehicle 20. Only vehicles traveling in front are taken into account inthe calculation of composite angular velocity ω_(c), whereas the signalsfrom stationary targets remain disregarded. In the simplest case, thelogic operation results in the formation of an average of all vehiclestraveling in front; i.e., the sum of relative angular velocities ω′_(i)of all vehicles traveling in front is divided by the number of vehiclestaken into account. Composite angular velocity ω_(c) is then compared toyaw rate 107 ₀ of subject vehicle 20 in a comparator circuit 42. Todetermine yaw rate ω₀, in the example shown a generally known yaw ratesensor 44 is used which measures the Coriolis force which appears duringa yaw motion of the vehicle, it being possible to also evaluate thesignals from the yaw rate sensor within the scope of a stabilityregulation for subject vehicle 20. Any systematic error (offset) of yawrate sensor 44 may be eliminated, if needed, by taking into account thesignals from a steering wheel angle sensor, a transverse accelerationsensor, a wheel speed sensor, and the like. The individual signals arealso checked for plausibility, and in non-plausible situations aconclusion is made as to the failure of the sensor. The signal from yawrate sensor 44 may also undergo low-pass filtering, preferably using thesame time constants as for the relative angular velocity signals.

In comparator circuit 42 a lane change signal LC is formed fromcomposite angular velocity ω_(c) and yaw rate ω₀ according to thefollowing formula:LC=−ω _(c)*ω₀/(ω_(c)+ω₀)  (1)

Lane change signal LC is sent to regulating device 12 which, bycomparing this signal to a suitable threshold value (symbolized by athreshold value switch 46 in FIG. 1), detects that a lane change bysubject vehicle 20 is occurring and then makes the appropriateadjustments in the distance regulation, in particular in thedetermination of the travel corridor.

FIG. 3 shows the course over time of a lane change of subject vehicle20, in this case from middle lane 16 to left lane 18. FIG. 4 shows thecorresponding time curve for yaw rate ω₀, composite angular velocityω_(c), and lane change signal LC.

At time t₀ the lane change has not yet begun, and the path direction ofsubject vehicle 20 remains parallel to the lane. Yaw rate ω₀ isconsequently zero. Relative angular velocity ω₁ of vehicle VEH1traveling directly in front in lane 16 is also zero. For vehicles VEH2and VEH3 in the adjoining lanes, however, this is true only if theirrelative speed is zero, i.e., if their respective distances to subjectvehicle 20 remain unchanged. On the other hand, if subject motor vehicle20 has a higher speed than vehicle VEH2 in lane 14, the (negative)azimuth angle ψ₂ for the latter vehicle increases in terms of actualamount, resulting in a negative relative angular velocity ω₂ Similarly,a negative relative angular velocity ω₃ likewise results for vehicleVEH3 in the adjoining left lane if this vehicle is faster than thesubject vehicle. Thus, without additional corrections a negativecomposite angular velocity would result during the formation of anaverage. To compensate for this effect, correction element 38 makes thefollowing correction:ω′_(i)=ω_(i) −V _(i)*ω_(i) /d _(i)  (2)

As a result of this correction, at time to composite angular velocityω_(c) obtained from the formation of the average is also zero. Lanechange signal LC produced according to equation (1) also has a value ofzero.

Between times t₀ and t₂, subject vehicle 20 veers left to the adjoininglane, and during this phase has a positive yaw rate which at time t₁ isat a maximum. The path direction of subject vehicle 20 also changes,corresponding to the yaw motion. Because the azimuth angle measured bylocation sensor 10 is based on this changed path direction, compositeangular velocity ω_(c) assumes a value equal to the yaw rate ω₀ in termsof actual value, but with an opposite sign. The product of the yaw rateand the composite angular velocity is therefore negative, and LCaccordingly assumes a relatively high positive value. At time t₂ the yawrate of subject vehicle 20 has again decreased to zero, and acountermotion is initiated for veering into the new lane. At this momentLC again returns to zero. In contrast, composite angular velocity ω_(c)still has a small negative value. This is due to the fact that the pathdirection of subject vehicle 20 at time t₂ is not parallel to the pathdirection of the vehicles traveling in front. For vehicles VEH1 and VEH2in particular, this results in a negative relative angular velocity evenwhen the relative speed has not become zero. Consequently, the zerocrossing of curve ω_(c) does not occur until a later time, so that LCtemporarily assumes negative values.

At time t₃, yaw rate ω₀ reaches a minimum and composite angular velocityω_(c) is at a maximum, and LC also increases again to a maximum. Allsignals then decrease again to zero until the lane change is completedat time t₄.

FIG. 4 shows that the lane change is marked by a characteristic S-shapedcurve for yaw rate ω₀ and by a characteristic “camel hump” for lanechange signal LC. Threshold value sensor 46 detects the start of a lanechange by the fact that lane change signal LC exceeds a specifiedthreshold value TH (at time t_(s) in FIG. 4). A short-term drop belowthis threshold value at approximately time t₂ indicates the midpoint ofthe lane change process, whereas another drop below threshold value THat time t_(e) indicates the end of the lane change.

For purposes of comparison, FIGS. 5 and 6 illustrate a driving situationin which no lane change takes place, but instead subject vehicle 20 anda vehicle VEH1 traveling in front travel out of a curve. The positionsof both vehicles at time t₁ are marked by bold lines in FIG. 5, whilethe positions at time t₂ are marked by thinner lines, and at time t₃ bydashed lines.

At time t₁ both vehicles are still in the curve. Subject vehicle 20 hasa positive yaw rate ω₀. However, for vehicle speeds which areapproximately the same, the positions of subject vehicle 20 and vehicleVEH1 relative to one another remain unchanged, so that composite angularvelocity ω₀ (which in this case only is indicated by ω′₁) has a value ofzero. Consequently, LC is also zero. This means that traveling through acurve is not erroneously interpreted by the system as a lane change.

Vehicle VEH1 traveling in front begins to travel out of the curvebetween points t₁ and t₂. Its relative angular velocity thereforedecreases, while yaw rate ω₀ of the subject vehicle remains constant.Lane change signal LC is therefore positive and assumes a flat maximumat t₂. However, since multilane freeways generally have very large radiiof curvature, the yaw rates and relative angular velocities which appearhere are very low, so that lane change signal LC remains below thresholdvalue TH.

As a variant of the described method, for signal ω₀ which is used tocalculate lane change signal LC it is not the actual measured yaw ratethat is used, but instead the instantaneously measured yaw rate minus amoving average from the previously measured yaw rates. When travelingthrough a curve at a constant actual yaw rate, the moving average wouldthen gradually approach the instantaneous yaw rate, so that signal ω₀would decrease essentially to zero. Consequently, lane change signal LCbetween times t₁ and t₂ in FIG. 6 would remain smaller. After time t₂the instantaneous yaw rate would decrease below the moving average, withthe result that signal ω₀ would be negative. Signal LC would thus alsobe negative between times t₂ and t₃. Thus, in this variant, thresholdvalue TH could be reduced to increase the sensitivity of the lane changedetection.

Regulating device 12 may react in different ways to the detection of alane change, at time t_(s) in FIG. 4, depending on the embodiment. Forexample, the ranging depth of the radar sensor may be reduced so thatregulating device 12 then responds to vehicles traveling in front onlywhen they are a very small distance in front of subject vehicle 20 andthere is a danger of imminent collision. Thus, if the path direction ofsubject vehicle 20 runs at an angle to the roadway, at time t₂ in FIG.3, irrelevant objects situated outside the lanes of interest areprevented from being evaluated.

In another embodiment, the original travel corridor is “frozen” when alane change is detected. This may be achieved by integrating measuredyaw rate ω₀ from time t_(s) forward. The integral then providesapproximately the angle of the instantaneous path direction of thevehicle relative to the direction of the roadway. If this angle issubtracted from measured azimuth angle ψ_(i), the result corresponds tothe subject vehicle for remaining in the original travel corridor.

Alternatively, the evaluation of the location signals at time t_(s) maybe limited to those vehicles that were present in the instantaneoustravel corridor prior to that time. This is possible because locationdata d_(i), v_(i), and ψ_(i) measured from one regulating cycle toanother for the same vehicle respectively differ only very slightly fromone another, so that the individual vehicles may be identified and theirmotions tracked. Additionally, a collision avoidance strategy may bepursued in which the system responds to vehicles, not previously takeninto account, when these vehicles are a very small distance in front ofsubject vehicle 20.

The signal of the turn indicator may also be included in the evaluation.In the situation shown in FIG. 2, when the driver's intent to make alane change by actuation of the turn indicator is detectable, the travelcorridor may be extended to a combination of both travel corridors 30and 34. At the same time, threshold value TH may be reduced so that theactual start of the lane change is detected earlier. At the detectedstart of the lane change, the extended travel corridor is then frozen,and finally, when the end of the lane change is detected at t_(e), thetravel corridor is narrowed to new travel corridor 34.

1. A method of detecting a lane change of a subject vehicle including alocating device adapted to use an angular resolution for locating atleast one vehicle traveling in front of the subject vehicle and a devicefor determining a yaw rate of the subject vehicle, comprising: measuringan angular velocity of the at least one vehicle relative to the subjectvehicle using the locating device; and comparing the angular velocity tothe yaw rate of the subject vehicle to form a lane change signalindicating the lane change.
 2. The method as recited in claim 1, furthercomprising: before the comparing of the angular velocity to the yawrate, correcting the angular velocity of the at least one vehicle toform another relative angular velocity which is independent of arelative speed of the at least one vehicle.
 3. The method as recited inclaim 1, further comprising: forming a composite angular velocity fromone of the angular velocity and the other angular velocity of aplurality of vehicles traveling in front of the subject vehicle, thecomposite angular velocity representing a relative change in an angle ofa combination of the plurality of vehicles; and comparing the yaw rateto the composite angular velocity.
 4. The method as recited in claim 3,wherein the composite angular velocity includes one of a weightedaverage of the other angular velocity, an unweighted average of theother angular velocity, a weighted average of the angular velocity, andan unweighted average of the relative angular velocity.
 5. The methodclaim 1, wherein the determining of the yaw rate includes evaluating asignal from a yaw rate sensor.
 6. The method as recited in claim 3,further comprising calculating the lane change signal according to aformula which gives a high positive value when the yaw rate of thesubject vehicle and one of the angular velocity and the compositeangular velocity have values different from zero and have oppositealgebraic signs.
 7. The method as recited in claim 6, wherein thecalculating of the lane change signal includes calculating across-correlation of the yaw rate of the subject vehicle using one ofthe angular velocity and the composite angular velocity.
 8. The methodas recited in claim 1, further comprising detecting a beginning of thelane change when the lane change signal exceeds a predeterminedthreshold value.
 9. The method as recited in claim 8, further comprisingreducing the predetermined threshold value when a turn indicator of thesubject vehicle is actuated.
 10. The method as recited in claim 8,further comprising detecting a completion of the lane change when thelane change signal falls below the predetermined threshold value for asecond time after the detecting of the beginning of the lane change.